If broadcasters (ABC, CBS, FOX) offered UHD programming, Cox would provide it via their CATV Distribution System and not as an Internet service. Meaning, no data cap with cable UHDTV. Unless, of course, if you'd be streaming UHD.
Bandwidth limits? Do you need more than 1000 Mbps? Are you waiting for Terablast Internet from Cox? I'm sure you meant Data Limits.
Internet Ultimate? Why would you need 300 Mbps to stream UHD? Or, more precisely, how many devices in your household would be streaming UHD...at one time? Even the greediest recommendation for UHD on 1 device is 25 Mbps. 2 devices 50. 3 devices 75. It would take 4 devices streaming UHD at one time just to reach one-third of 300 Mbps.
All Cox Internet plans cap at 1 TB, so that wouldn't dictate your choice. You need to determine the number of devices that'd be streaming at one time. If you have 1 device, you only need Cox Essential 30. 2 devices could use Preferred. 12 devices would be Ultimate.
Fighting the Data Limit is another story.